1. Field of the Invention
This invention relates to a high-bulk, wood containing printing paper offering high brightness, very low density, high flexibility (low stiffness), and excellent printability.
2. Description of the Related Art
High bulk (low density) is one of important qualities required by paper products of late. Responding to the growing pro-environmental sentiment, the users are demanding paper products that offer lower weight, or lower density, while maintaining sufficient thickness, in order to achieve more effective utilization of paper pulp produced from forest resource.
One traditional method for reducing paper density (increasing paper bulk) is to use high-bulk pulp. In general, wood pulp is used widely in paper production. It is known that ground pulp produced by grinding wood material using a grinder, or refiner mechanical pulp produced by refining wood material using a refiner, or yet mechanical pulp such as thermomechanical pulp, offers stiffer fibers and is therefore more effective in reducing paper density, compared with chemical pulp produced by extracting a reinforcing substance called lignin from wood fibers through chemical processing. However, today's mechanical pulp is mostly produced from softwood trees containing large amounts of lignin, which makes it difficult to produce pulp offering high brightness. A number of issues have therefore been encountered in the effort to produce a high-bulk paper offering high brightness.
Among various types of mechanical pulp, softwood thermomechanical pulp has relatively long and stiff fibers. Therefore, paper made from such pulp had poor formation and lower smoothness, and also caused ink impression problem during offset printing. Paper made from softwood thermomechanical pulp containing stiff fibers also resulted in lower foldability—a factor that affects offset printing efficiency—and made it difficult to turn pages of printed/bound books. Regarding the use of softwood thermomechanical pulp, therefore, improvement of flexibility, or reduction of flexural stiffness, of printing paper remained an issue.
To solve the above problems, the applicant filed a patent application regarding a technology used to produce from specific hardwood trees a mechanical pulp offering high bulk, high brightness and high relative scattering coefficient (Japanese Patent Application Laid-open No. 2003-27385). The applicant also filed a patent application regarding a printing paper containing hardwood mechanical pulp, which offers low density, high brightness, high opacity and excellent printability (Japanese Patent Application Laid-open No. 2003-49386). However, when what happens to the strength, printability and other properties of paper as a result of increasing paper bulk are put into overall perspective based on the use of hardwood mechanical pulp, these conventional technologies can only achieve a paper density of around 0.50 g/cm3 at the lowest for printing paper fit for practical use, and therefore further increase in paper bulk (reduction of density) remained difficult.
Another known method for reducing paper density is to use a bulk-increasing agent. Known bulk-increasing agents include a bulk-increasing agent for paper containing specified alcohol and/or its polyoxyalkylene adduct (PCT International Publication No. 98/03730), nonionic surfactant (Japanese Patent Application Laid-open No. 11-200283) and bulk-increasing agent for paper consisting of polyvalent alcohol and fatty acid ester compound (Registered Japanese Patent No. 2971447), and a technology that applies these bulk-increasing agents for paper to paperboard is also disclosed (Registered Japanese Patent No. 3041294).
As for the bulk-increasing agent based on a non-surfactant, a bulk-increasing agent for paper that contains at least one type of compound selected from a cationic compound having a specified structure, amine, acid salt of amine, or amphoteric compound is disclosed (Japanese Patent Application Laid-open No. 11-269799). A bulk-increasing agent of fatty acid polyamide polyamine type is also known. Also known is a compound with a water separation degree of 4% or more that provides the effect of increasing two or more paper qualities including bulk, brightness and opacity (Registered Japanese Patent No.3283248). Additionally, a method for producing a pulp sheet is also disclosed, where the method uses a compound with a water separation degree of 4% or more that provides the effect of increasing one or more paper qualities including bulk, brightness and opacity (Japanese Patent Application Laid-open No. 2003-105685).
Efforts to increase paper bulk using these known bulk-increasing agents cannot reduce the density of obtained printing paper to levels below 0.50 g/cm3 or so, which is similar to the case when hardwood mechanical pulp is used as mentioned above, and therefore ways to further reduce paper density are needed.
Even if paper bulk can be successfully reduced and a low-density paper is obtained, when offset printed such paper would cause “bleeding,” or oozing out of ink onto the back face over time. This problem, which affects printing quality, became a major issue in the effort to reduce paper density.
Based on the above background, development of a high-bulk, wood containing printing paper offering high brightness, low density, high flexibility (low stiffness) and excellent printability, while also preventing bleeding, has been awaited.